1852. ] OF THE ROYAL INSTITUTION. 165 
upwards, and draw in water to lubricate its gills, and reject it when 
it has served the purposes of respiration. The water at Mobile is 
usually fresh, but sometimes brackish. Sir Charles examined lately 
the Woolwich beds with Mr. Morris, and they verified Mr. De la 
Condamine’s observations, observing there several dozen specimens 
of the Cyrena tellinella in an erect position. From this circumstance 
the Lecturer infers, that a body of fresh or river water had been 
maintained permanently on that spot during the Eocene period, and- 
the presence of rolled oysters in the associated pebbly layers, with other 
marine shells, mixed with species of Melanopsis, Melania, Cerithium 
and Neritina demonstrate that the sea occasionally invaded the same 
area. To an overflow of the pebbly sand in which the Cyrene lived 
by salt water, may probably be attributed the poisoning of the mollusks 
which left their shells uninjured on the spot where they had lived. 
The stratum called ‘‘ the shell-bed,’’ which contains at Greenwich, 
Woolwich, Upnor near Rochester, and other places, a great mass 
of fresh water, brackish-water and marine shells, especially oysters, is 
observed everywhere to underlie the great pebble-bed. Its mode 
of occurrence implies the entrance of one or more rivers into the 
Eocene sea in this region. Other rivers draining adjoining lands 
are indicated by a similar assemblage of fluvio-marine fossils near 
Guildford and at Newhaven in Sussex. The vicinity of land to the 
South and West of Woolwich is shown by the occurrence at New 
Cross, Camberwell, and Chelsea of Paludina and Unio in strata 
evidently a prolongation of the Woolwich beds, and by fossil leaves 
of dicotyledonous trees and layers of lignite in some of those loca- 
lities. On the other hand at the junction of the ‘* London Clay,” 
and the subjacent “ plastic clays and sands,” when followed in an 
opposite or easterly direction towards Herne Bay and the Reculvers, 
all signs of the freshwater formation disappear, and the pebble-bed 
is reduced to a thin layer, often a foot or a few inches in thickness. 
The origin of this shingle may have been chiefly due to the action of 
waves on a sea-beach. Its accumulation in great force at certain 
points where freshwater shells abound, seems to imply the entrance of 
rivers into the sea, which brought down some fiints, and arrested the 
progress of others travelling as beach pebbles along a coast line, 
in a certain direction determined by the prevailing currents and 
winds. The spreading of the pebble-bed over a wide area may 
be accounted for by supposing a gradual subsidence of land, and the 
continually shifting of the coast-lines upon which shingle accumulated. 
This same subsidence is required to explain the superposition of the 
London Clay, a deep-sea deposit to the Blackheath or Woolwich beds 
which are of shallow water or littoral origin. One of the rivers of 
the Lower Eocene period swept into the sea at Kyson near Wood- 
bridge in Suffolk the bones of a monkey of the genus Macacus, of a 
marsupial quadruped allied to the opossum, of a Hyracotherium, 
and other mammalia, which have been determined by Professor Owen, 
and which throw light on the inhabitants of the land, at an era an- 
tecedent to the deposition of the London Clay. 
